The question is really all in the heading: is it possible for a macroscopic organism to subsist entirely on rocks and still gain enough calories to survive? I know of Lithotrophs but all known lithotrophs are bacteria or archaea.

My idea at present is that existing lithotrophs were incorporated into the gut bacteria of animals that swallow stones to help digestion, and as they eat part (or all) of the stone they thrive and also aid the creature eating the stone. So I've figured out how and why they eat rock in the first place; could any macrofauna get enough calorie for life purely from eating rocks? If not, how many calories could they get? (So I can figure out what else they eat to make it up)

If it matters, the creatures in question are flightless birds around the size of a grey heron.

$\begingroup$Welcome to the site, swinefish. I've made a few small edits to your post. I also removed a tag another user added that put additional constraints on answers you hadn't included. If you feel the hard-science tag applies to your question, feel free to add it back in.$\endgroup$
– FrostfyreApr 14 '16 at 12:06

3 Answers
3

Is it ok if I have my stomach on the outside? It's what humans do when they cook after all.

Let's invent lithotroph agriculture. You're macroscopic organism eats "rocks" by running over them like a lawnmower, just a little slower. As it passes it takes it's harvest while picking lithotrophs up from consumed rocks and laying them down on fresh rocks. Leaving a trail of devastated rock surface to mark its passing.

Key here is it doesn't drag them forward. It pushes them sideways. Leaving behind lithotrophs to grow on fresh rock until it returns. Thus making your organism territorial.

$\begingroup$Hmmm. Not quite what I was thinking of, but certainly an interesting perspective and potential use.$\endgroup$
– swinefishApr 14 '16 at 8:40

$\begingroup$Focus was on solving the problem of too little energy. Something else I should focus on?$\endgroup$
– candied_orangeApr 14 '16 at 8:44

$\begingroup$No, I just meant the type of creature wasn't what I had in mind. But the more I think about it the more interesting it becomes.$\endgroup$
– swinefishApr 14 '16 at 8:59

$\begingroup$To most people watching this it would look the birds are eating the rock as they scrape up the little bugs off the old rock and poop them onto the new.$\endgroup$
– candied_orangeApr 14 '16 at 9:02

$\begingroup$Yeah. It is really nice, and allows similar organisms to be figured out without too much difficulty$\endgroup$
– swinefishApr 14 '16 at 9:05

Lithotropes do not literally eat rocks. Rocks are a mix of inorganic minerals that have very little to no available chemical energy, which is why there is so much of it. If minerals were an actual food source, rock-eating organisms would be the dominant life form on Earth by a large margin, if organic life existed at all.

Instead they consume non-organic matter, i.e. matter that does not contain organic compounds, but only those that still contain a usable energy potential, such as ions, salts, or hydrogen.

The main problem for lithotrophes is that their source of energy has a very low density. A macroscopic organism would require impractically high amounts of food to survive and thrive simply because of its mass. One kg of microbes can spread out much further and be more efficient at gathering food than one macroscopic organism of the same weight.

Large organisms are also less efficient than microscopic ones overall, because an increasing share of its body is devoted to purposes other than finding and processing food, such as structural cells, nervous system, propulsion, reproduction, and so on.

Macroscopic lithotrophes might be possible on a world that contains an abundance of such energy sources, and where those energy source would be constantly produced by other processes so that those creatures have enough time to evolve, but it certainly wouldn't be possible on Earth.

Perhaps alkali metal or somesuch? There aren't any rocks like that. Think about why: any minerals that contained potential energy easy to liberate spontaneously would have done so already and not formed long-lived minerals.

Your only option is to eat several different minerals that together allow a chemical reaction that produces energy. Ask a chemist about what is possible, if anything. I can imagine having the animal catlyize a reaction that produces energy, but I don't know the magnitude of what specific reactions are possible. Ask a chemist or make up something. I'm afraid there isn't anything real that would give enough power.